It is a generally known requirement of certain computer graphic output devices to convert the resolution of digital input images prior to outputting them at a predefined output resolution (pixel density or raster density). Specifically, printer systems capable of printing image data and/or character data have become widely used. Furthermore, printer systems of this type currently provide high resolution, e.g., 600 spots per inch (spi) output capabilities which, in many cases, is higher than the input resolution available. This results in a situation where the performance of the printer may not be used to the maximum. For example, many existing input scanners and personal computer software packages operate at a medium image resolution of 300 spi, resulting in either an incompatibility with higher resolution printers, or underutilization of the output capabilities of high resolution printers.
For these reasons, current technology is focused upon means for conversion of image data of one resolution to an image of a second resolution. It is a commonly known practice to implement this type of conversion through simple pixel and raster level operations, such as pixel doubling. Alternatively, more elaborate methods have been developed to facilitate this type of conversion.
Takano et al., U.S. Pat. No. 4,632,579, discloses a method of increasing print resolution for a pin matrix printer by mechanically altering the relative position of the print head with respect to the paper during subsequent printing passes to increase the output resolution of the printer. Also disclosed is a means for controlling the activation of printing pins using a microprocessor based interpolation scheme to synthesize a secondary dot matrix pattern based upon a primary dot matrix pattern contained in memory for the associated character or pattern.
Nakajima et al., U.S. Pat. No. 4,841,375, teaches an image resolution conversion apparatus for converting the pixel density of image data, comprising an input means for receiving image data, a selection means to determine the appropriate pixel-density conversion ratio, and a pixel-density converting means. The pixel-density converting means comprises, inter alia, a conversion-pixel density operating means for calculating a conversion-pixel density based on the input pixel data and a binary encoding means for subsequently applying a threshold to the conversion-pixel density value and generating a conversion-pixel binary output. Also disclosed is a threshold-value setting means which dynamically adjusts the threshold value to be applied within the binary encoding means on the basis of pattern detection applied to the original or reference image data.
Others have proposed template matching techniques for more precisely controlling the size, positioning and number of pixels that are printed on a xerographic photoreceptor to render bit-mapped images.
Walsh et al., U.S. Pat. No. 4,437,122, describes a method of enhancing the resolution and quality of characters of a system receiving video display pixel information and providing hard copy output. The system accomplishes this by storing at least three successive lines of video data in successive parallel connected shift registers, applying the output of the shift registers to a decoder, and generating driving signals for a printer head. The decoder compares the pixels on the same line as well as in preceding and succeeding lines that surround each specific input pixel to generate the printer head driving signal according to whether straight or curved line segments are to be formed. Enhancement of the central pixel may be determined by progressively examining an ordered table of matches to find an equivalent image and its related enhancement.
A particular problem arises in the resolution conversion of bit-map images where the images have been previously optimized for a specific printing technology. "Write-black" refers to an electrophotographic printing process in which the portions of the image that will attract black toner for eventual transfer to a copy substrate are discharged with an exposure device, such as a laser. By contrast, "write-white" refers to an electrophotographic printing process in which the portions of the image that will not receive toner are discharged with an exposure device.
Thus, for example, images may be optimized for write-black printer technology, which tends to thicken lines because the optical input has a width greater than the pixel width, and subsequently transmitted to a write-white printer, which tends to thin lines because the discharged areas adjacent to lines areas spread into the domain of the line. Such images will appear undesirably lighter, because the correction for the expected write-black printer thins the lines m the image. When such a corrected image is sent instead to a write-white printer, the result will be lines that are too thin. Single pixel lines will tend to be invisible. Another effect that might be present in an optimized image might be halfbitting, which may not be required in a higher resolution image.
Mailloux, U.S. patent application Ser. No. 07/588,125, now U.S. Pat. No. 5,410,615 discloses a method for conversion of binary image data into representation binary image data at an integer-multiple higher resolution depending upon the capabilities of the printer, e.g., 2x, 3x, 4x, etc. In the process of conversion of binary image data originally generated at a lower resolution and optimized For a particular printer technology, an amount of line thickening, or dilation, is provided which corrects the bit-map for reproduction in a printer having line thinning characteristics, such as a write-white printer. The dilation function may be performed by the application of state rules to the magnification which tend to favor line thickening. The expanding magnification method, illustrated by the 2x method as described in Mailloux, improves printing of fine lines, but tends to lighten reverse printing and fill in shadow areas.
It is an object of this invention to convert bit-map images, originally generated at a first resolution and optimized for a particular printer technology, to a second resolution for a printer having different characteristics, providing controlled dilation, wherein the controls assist in retaining discernible features of fine lines and graphic images.